As a method for producing high-purity polycrystalline silicon used as a raw material for monocrystalline silicon for production of a semiconductor, a Siemens method and a fluidized bed reactor method are known. The Siemens method is a method in which a raw material gas containing chlorosilane is contacted with a heated silicon core, and polycrystalline silicon is grown on the surface of the silicon core by a CVD (Chemical Vapor Deposition) method. The fluidized bed reactor method is a method in which monosilane or trichlorosilane as a raw material is fed and vapor-deposited in a fluidizing gas to obtain granular polysilicon.
In production of polycrystalline silicon, such a phenomenon is known that the quality of polycrystalline silicon is reduced by an impurity gas being discharged from an inner wall surface of a reaction vessel used for a deposition reaction of silicon and undesirably taken into the polycrystalline silicon. In the case where a steel reaction vessel is used, when the temperature of an inner wall surface of the steel reaction vessel is not less than 400° C., that inner wall surface of the steel reaction vessel which contacts a process gas prepared by diluting a silicon raw material gas such as trichlorosilane with hydrogen gas is gradually corroded. As a result, not only component elements of the steel that constitute the inner wall surface of the steel reaction vessel, but also impurity elements contained in the steel are discharged into the reaction atmosphere. If these impurity elements are dopant impurity elements such as phosphorus, boron, aluminum, and arsenic that act as an acceptor or a donor in silicon crystal, the impurity elements result in remarkable reduction of the quality of polycrystalline silicon.
In consideration of such problems, Japanese Patent Laid-Open No. 8-259211 (Patent Literature 1) discloses a technique for obtaining deposited high-purity silicon by depositing silicon within a reaction vessel formed with a material that hardly outgases.
Specifically, based on a knowledge that a heat-resistant alloy containing not less than 28% by weight of nickel hardly outgases at a temperature of not more than 600° C., the decomposition and reduction reaction of silanes are performed within a reactor vessel having an inner wall comprising a heat-resistant alloy containing not less than 28% by weight of nickel, thereby to further increase the purity of polycrystalline silicon to be obtained. Examples of the above-described “heat-resistant alloy containing not less than 28% by weight of nickel” include Incoloy 800, Inconel 600, Inconel 601, Incoloy 825, Incoloy 801, Hastelloy B, and Hastelloy C.